1. Field of the Invention
The present invention relates to the communication of data over a local area network (LAN) and, more particularly, to a system for increasing the data communication capacity of an existing LAN that uses a shielded twisted wire pair as its transmission medium.
2. Description of the Prior Art
A local area network (LAN) is a data communication system that allows a large number of sub-systems to communicate over a common data bus. Each of the subsystems is individually connected to the common data bus and a standard protocol is adopted by each sub-system so as to prevent or minimize when more than one sub-system transmits on the common data bus at the same time. Generally, each of the sub-systems transmits data on the common data bus at the same rate, or frequency.
The data communication frequency of the LAN is determined by a number of factors, one being the medium of the common data bus. One of the more popular types of media used today, due to its low cost of installation and service, is a shielded twisted wire pair. The shielded twisted wire pair allows each of the sub-systems to transmit signals differentially over the common data bus wire pair. As is commonly known, one of the most important aspects of a differential medium is its Common Mode Rejection, which allows the medium to be used in very noisy environments. Accordingly, if noise, that is analogous in both amplitude and phase, is injected into both wires of the shielded twisted wire pair, a data transmission from one of the sub-systems will not be affected by that noise. Similarly, if a signal, that is also analogous in both amplitude and phase, is injected into both wires of the shielded twisted wire pair, a data transmission from one of the sub-systems will not be affected by that signal. To date, this important aspect of the shielded twisted wire pair differential medium has not been utilized to its fullest extent.
As previously discussed, the LAN allows a large number of sub-systems to communicate over a common data bus at a specified frequency by adopting a standard protocol. However, it is often desirable or required to have these same sub-systems communicate at a higher data communication rate or by a different protocol. Since the shielded twisted wire pair provides only a single data channel with no selectivity, an additional data channel would be required to accommodate such a desire or requirement. The present invention utilizes the Common Mode Rejection of the shielded twisted wire pair differential medium to provide an additional data channel, thereby increasing the overall data communication rate and allowing a different protocol to be used by the subsystems.